1. Field of the Invention
The subject invention is directed to a fuel injection system for industrial gas turbines, and more particularly, to a fuel injection system for atomizing industrial grade fuels in gas turbines during ignition.
2. Background of the Related Art
Gas turbines are employed in a variety of industrial applications including electric power generation, pipeline transmission and marine transportation. A common problem associated with industrial gas turbines is the difficulty associated with initiating fuel ignition during engine startup cycles. Moreover, during startup, the fuel must be presented in a sufficiently atomized condition to initiate and support ignition. However, at engine startup, when the engine is gradually spooling up, the fuel and/or air pressure needed to atomize the fuel is generally unavailable.
A broad range of fuel injection devices and methods have been developed to enhance fuel atomization during engine ignition sequences. One approach has been to employ pressure atomizers, which, in order to operate at the low fuel flow rates present at ignition, have small fluid passages that generate the high fuel velocities needed to effect atomization. However, these small passages are susceptible to fuel contamination and carbon formation, and thus limit the service life of the fuel injector with which they are associated.
In contrast, large aircraft engines can start on conventional pure air-spray injectors and benefit from the long service life experienced with airblast atomizers which utilize the kinetic energy of a flowing air stream to shatter a fuel sheet into fine droplets. This is possible because a jet aircraft engine uses lighter aviation fuel, and typically has an auxiliary power unit that can spin the engine to a sufficiently high speed to produce the differential air pressure required to start an airblast atomizer. Most airblast atomizers in use today are of the prefilming type, wherein fuel is first spread out into a thin continuous sheet and then subjected to the atomizing action of a high velocity air flow.
Typically, at ignition, airblast atomizers have difficulty atomizing heavy viscous industrial fuels, such as diesel fuel. This is because industrial grade fuels such as DF-2, as compared to lighter less viscous fuel such as aviation grade Jet-A, require a greater differential air pressure to effect atomization.
It would be beneficial to provide a fuel injection system for industrial gas turbines that is adapted and configured to efficiently atomize industrial grade fuels under the relatively low air pressure conditions that exist during engine ignition. There is also a need in the art for a low cost fuel injector for use in conjunction with industrial gas turbines that does not have the type of structural features that are susceptible to fuel contamination and carbon formation, as is found in pressure atomizers.
The subject invention is directed to a low-cost airblast fuel injector for use in conjunction with industrial gas turbines, and more particularly, to a fuel injector for use in conjunction with a system and method for atomizing industrial grade fuel issuing from the injector. The term airblast is used herein to describe the way in which the fuel issuing from the nozzle is atomized, i.e., by way of the energy transferred to the fuel from an air stream rather than by way of the energy of the fuel flow itself.
The fuel injector of the subject invention includes an elongated tubular body having at least first and second concentric tubes separated from one another by a helical spacer wire so as to define a annular fuel passage therebetween configured to issue a swirling extruded fuel film that is easily atomized by an intersecting air stream. Preferably, the first tube is an outer tube and the second tube is an inner tube, and the helical spacer wire is supported on an exterior wall of the inner tube, by means such as brazing or the like.
The subject invention is further directed to a fuel nozzle which includes a nozzle body having a discharge section with an interior chamber. The discharge section has a fuel inlet port formed therein for admitting an extruded fuel film into the interior chamber thereof. The discharge section also has an air inlet port disposed adjacent to the fuel inlet port for directing an air stream into the interior chamber of the discharge section so as to intersect the fuel film at a predetermined angle to effect atomization of the fuel film.
The nozzle assembly further includes an airblast fuel injector constructed in accordance with the subject invention which communicates with the fuel inlet port. The fuel injector has an elongated tubular body including inner and outer concentric tubes that are separated from one another by a helical spacer wire so as to define a fuel passage therebetween. In accordance with the subject invention, the air inlet port formed in the discharge section of the fuel nozzle is oriented and configured in such a manner so as to direct air at the fuel film at a predetermined angle of incidence so as to atomize the fuel flow.
The subject invention is further directed to a nozzle assembly which includes a nozzle body having a discharge section with an interior chamber that defines a central axis. An annular swirl plate is disposed within the interior chamber of the discharge section. The swirl plate has a plurality of circumferentially spaced apart air channels formed therein for directing air radially inwardly in a plane extending generally perpendicular to the central axis of the interior chamber. In addition, the swirl plate has a plurality of circumferentially spaced apart fuel inlet ports formed therein. Each fuel inlet port is adapted to admit an extruded fuel film into the interior chamber of the discharge section at a location that is adjacent to a radially inner end of a corresponding air channel. As a result, the air flowing through each channel intersects a corresponding fuel film at a predetermined angle to effect atomization of the fuel film. Preferably, each fuel inlet port is aligned with the central axis of the interior chamber of the discharge section such that the air flowing through each channel intersects the fuel film issuing from each fuel inlet at a 90 degree angle.
The fuel nozzle further includes an airblast fuel injector constructed in accordance with the subject invention which communicates with each fuel inlet port of the swirl plate. Each fuel injector has an elongated tubular body including inner and outer concentric tubes that are separated from one another by a helical spacer wire so as to define a fuel passage therebetween.
The subject invention is also directed to a method of atomizing fuel which includes the initial step of providing a fuel injector having an elongated tubular body including inner and outer concentric tubes that are separated from one another by a helical spacer wire so as to define a fuel passage therebetween. The method further includes the steps of flowing fuel through the fuel passage of the tubular body so as to extrude the fuel flow, and intersecting the extruded fuel flow exiting the fuel passage of the tubular body with an air flow at a predetermined angle of incidence so as to atomize the extruded fuel flow.
In accordance with the subject invention, the extruded fuel flow exiting the fuel passage is intersected with an air flow at an angle of incidence ranging from about parallel with an axis of the tubular body to perpendicular to the axis of the tubular body. The method also includes the steps of flowing a fluid such as air, fuel or water through the inner tube so as to modify the spray characteristics of the injector, and providing the air flow from turbine compressor discharge air or from an auxiliary air compressor.
An important aspect of the low-cost fuel injector of the subject invention that sets it apart from existing fuel atomization devices known in the art, such as airblast atomizers and pressure atomizers, is the absence of precision machined components needed to produce a fine spray of atomized fuel. Moreover, the fuel injector of the subject invention does not have small flow passages consisting of fine slots, vanes or holes that swirl the fuel flow and produce a thin film that can be atomized. Precision machining of such passages is generally required to ensure that all of the injectors utilized with an engine flow at the same fuel flow rate, spray angle and droplet size distribution.
These and other aspects of the subject invention and the method of using the same will become more readily apparent to those having ordinary skill in the art from the following detailed description of the invention taken in conjunction with the drawings described hereinbelow.